DBFS is an internet abbreviation for "don't be fucking stupid." Its primary use is to admonish someone for a stupid question or their lame behavior.

DBFS is often preceded by a hashtag (regardless of medium): #dbfs

Allison: What are you doing on Sunday night?
Liz: It's Sunday, we're watching "Game of Thrones." DBFS!
Allison: I haven't started watching it. What is it about?
Liz: Winter is coming. Just watch it. DBFS!